Apparently I’ve heard someone say that when they were going far into their exploration of the astral plane, they encountered Doctor Strange and he guided them back to their body.
Now let’s suppose this story is true. If so, then is it possible that because of the MCU, a lot of people made the thoughtform of Dr. Strange and he’s now an independent astral entity? Has anyone had any encounters with fictional characters in the astral plane?
Depends on how you view reality. If your views are anchored in a perspective that encompasses interdimensional planes, multplie time lines and thus an expanding multiverse then yes it is entirely possible to run into entities that on this what is commonly perceived as the local time space considered fictional.
When leaving this sphere to traverse the inner or outer planes of reality, by this i mean outside the local immediate environment ie (astral, mental, soul planes, infernal empire, arezura, the 9 worlds of norse tradition to name a few) you have left linear time space and entered into planes which may very well have connections into other realities than the local one you left.
At this point the question becomes what practical uses can be gained from this that can result in TANGIBLE PRACTICAL change in your life and the lives of those around you.
Cause unless this 
occurs it is simply an experience. While experiences can be fun and insightful, unless it results in practical long term results it is pretty much just a mental and energetic exercise.
I think in astral plane any thing is possible because I have come across posts where people said they evoked video game characters and comic characters. All those characters exist in mind and images like books and screens, so I think you can see them easily in astral plane. Other than that I don’t think Dr. Strange or any one is an independent concrete entity.
The way I confirm is only when I come across something or someone I haven’t heard or read before anywhere that it has a separate existence.
Put it in a different perspective.
Would they have been in big trouble when he didn’t intervine with the issue?
In other words, would they’ve been stranded somewhere they don’t belong / felt lost?
I think that’s the more importend part here.
And weather you count MCU as real enough to be interfeering with our works or not,
the mind can very easily interpret a thing as something completely different,
if that suits the understanding of the person.
Sincerely,
¥’Berioth
For the Request wether MCU can interfeer with our forms of Sorcery:
Maybe this helps you answer that question for yourself, individualy. 
It’s MCU Pantheon and Hyrachie Explenation.
Sincerely,
¥’Berioth
By the Way,
i found this little piece about Dr. Stranges Ascend to Godhood.
And i must say some things really ring a bell when looking into it.
For example the void.
Sincerely,
¥’B